1. Field of the Invention
The present invention relates generally to Digital Rights Management (DRM) service, and more particularly, to a DRM service method and apparatus applicable to adaptive streaming service.
2. Description of the Related Art
DRM service is technology for continuously managing and protecting the intellectual property rights for digital content using encryption technology. In other words, this service is technology for securely delivering a variety of content from a Content Provider (CP) to a user, and preventing the user from illegally distributing the provided content. The DRM technology may protect the information throughout the entire process including creation, distribution, use and disposal of digital content, and may protect the user's rights not only in the online environment but also in the offline environment. Currently, various different DRM systems are available in the market. For example, such DRM systems may include an Open Mobile Alliance (OMA) DRM system, a Marlin DRM system, and a Widevine DRM system. These DRM systems are common in encrypting digital content before delivery and granting an access right to the digital content, i.e., in using a license, but different not only in the language expressing the license or the format thereof, but also in the format of messages used to run DRM service. Therefore, if the DRM system used by the CP providing DRM content is different from the DRM system supported by a user terminal, incompatibility-related problems may occur.
The digital content is provided to the user terminal by various different delivery schemes. Among them, streaming service is service that provides digital content data in real time. The DRM technology may be applied to this streaming service, and may also be applied to adaptive streaming service that automatically adjusts a streaming rate or bit rate in order to optimize throughput depending on the access environment.
The adaptive streaming service is technology for optimizing a bit rate by analyzing the user's network environment. In this technology, a user terminal selects and plays, depending on its situation, segments of content divided into small segments based on a Hyper Text Transfer Protocol (HTTP) protocol, allowing the user to enjoy the content seamlessly or without delay.
The terms used for the adaptive streaming service may be defined as follows.                Content: creations such as movies, music and news, and their storage form        Content component: components constituting content (ex: video, audio, and subtitles)        Representation: representation form of content. For example, even though low-quality and high-quality representations may exist, their contexts are the same.        Media Presentation Description (MPD): it includes information that a client (or user terminal) receiving and processing content should detect in advance, such as time location, Uniform Resource Locator (URL) and size of the content corresponding to the files obtained by segmenting the entire content into segments with a proper size.        
The adaptive streaming service provides representations obtained by encoding the same content and content components with different conditions, such as network bandwidth, resolution, and codec. The adaptive streaming service segments the entire length of content into several equal periods, segments again the periods into segments having a sufficiently short time (e.g., 1 to 10 seconds) (segmentation), and provides MPD information (serving as a manifest file) specifying a file name, a sequence number, a start time, a length, etc. of each segment.
To make it possible to properly decrypt their representations, segments consist of initialization segments having information for decoder configuration and media segments including segmented content components.
A user terminal first receives MPD information, determines which representation has been provided, determines the performance and capability of the client device, and the network speed, selects a proper representation, downloads their segments in sequence, and plays the downloaded segments.
If changeable variables such as the network bandwidth vary, i.e., if the network speed increases, the user terminal selects and plays segments of a representation made with a broader network bandwidth. As a result, though the quality varies with the network speed, the playback continues seamlessly.
Generally, when DRM technology is applied to the adaptive streaming service, a maximum of one DRM system may be supported in a fixed manner per streaming service or streaming content. A content server providing streaming service transmits MPD information expressed in an Extensible Markup Language (XML) to a user terminal.
The MPD information, basic information about content in service, includes information about a type of the service, a valid period of the service, etc. The MPD information may further include information about one or more representations. The representation information indicates whether DRM is applied to the streaming service, and includes identification information for content provided by the streaming service and address information based on which content may be acquired.
Based on the MPD information, the user terminal selects representation information having a proper bit rate taking into account a network bandwidth in each period of streaming data.
FIG. 1 illustrates an example of representation information defined in an MPD according to the prior art. Referring to FIG. 1, the representation information includes bandwidth information 10 to which the representation is applied, content protection information 20 indicating whether DRM is applied, and information about the applied DRM system, and segment information 30 indicating identification information and acquired address information for each segment constituting the content provided by the streaming service. In FIG. 1, it can be seen that information about one DRM system is shown as “OMADRM” in the content protection information 20.
The user terminal determines a segment Uniform Resource Identifier (URI) included in the representation information of FIG. 1 and determines possibility of content protection. In the case of DRM-applied data, the user terminal additionally acquires a license using the representation information, decrypts the encrypted segments, and plays the content.
However, when one DRM system is represented in a representation as described above, it is impossible to support multiple DRM systems. For example, if protection systems having different security levels are required depending on the bit rate type (for high-resolution image, low-resolution image, etc.), even though multiple DRM systems may be used, it is not possible to support multiple DRM systems.